This invention relates to a pulse-width modulation inverter for driving an AC motor or the like, and in particular to a wave generation circuit for generating waves used to form driving signals for switching elements in a pulse-width modulation inverter.
A conventional wave generation circuit for a pulse-width modulation inverter used to generate waves of three different phases is illustrated in FIG. 1, generally designated by reference numeral 10. The generated waves form a plurality of reference sine waves. The reference sine waves are then compared with a saw tooth carrier wave to form pulse width modulated waves which serve as driving signals for the inverter, as is well known in the art. In the figure, an address signal S.sub.1 is provided for first, second, and third phase Read Only Memories (hereinafter called ROM's) 12, 14, and 16 which respectively provides as outputs the waveform data S.sub.21, S.sub.22, and S.sub.23 for the three differently phased waves generated by the circuit 10. The ROM's 12, 14, and 16 have been respectively provided with stored waveform data Da, Db, and Dc for the generated waves over memory addresses 0-m. As can be seen from FIGS. 2(a), 2(b), and 2(c), the value of Da at a given address is equal to the value of Db at the address + m/3, where m is the total number of address in each ROM, and similarly, the value of Db at a given address equals the value of Dc at that address + m/3. The address signal S.sub.1 has a desired frequency corresponding to the output frequency of the inverter. In response to the address signal S.sub.1, the ROM's 12, 14, and 16 provide as outputs the three-phase waveform data S.sub.21, S.sub.22, and S.sub.23 separated by a phase difference of 120 degrees, as shown in FIGS. 4(b), 4(c), and 4(d).
The conventional circuit illustrated in FIG. 1 has the disadvantage of high cost due to the necessity of expensive memory elements (for example, ROM's) corresponding in number to the number of phases used. Also, with the conventional circuit, it is difficult to make modifications of the waveform data.
Accordingly, it is an object of the invention to provide a wave generation circuit for a pulse width modulation inverter in which the number of expensive memory elements used is as few as possible.